1. Field of the Invention
The present invention relates to a microwave oven with an inverter controlled power source and, more particularly, to a microwave oven being capable of adjusting the power to the magnetron at a level selected by the user.
2. Description of the Prior Art
In general, a microwave oven with an inverter control power source produces the high voltage power of high frequency (some tens of Kilo Hertz) and the cathode filament current for driving the magnetron. Simultaneously, the inverter controlled power source detects the high voltage power supplied to the magnetron and controls the high voltage power and the cathode filament current so that the high voltage power stays within an allowable range. Both the high voltage power and the cathode filament current are supplied to the magnetron through the high voltage coil of high voltage transformer and the driving coil for the cathode filament. The high voltage coil and the driving coil for the cathode filament are directly connected to a magnetron by the connecting wire, or a ferrite bead is provided around the connecting wire so as to provide an inductance for limiting the power supplied to the magnetron.
However, the magnetron generates heat by the power loss of the microwave as well as by receiving the reflected microwave due to the impedance unmatching with respect to the load inside the oven. The cooling fan is employed to cause the heated magnetron cool down so as to keep the magnetron working stably at a suitable temperature, hereinafter referred to as a "working equilibrium temperature".
According to FIG. 7, I.sub.1 f and I.sub.1 l in dotted lines show the cathode filament currents when the microwave oven with a conventional ferrite bead is operated under the maximum rated power and under the minimum rated power with slight load, respectively, and I.sub.0 f and I.sub.0 l in dashed lines show the cathode filament current when the microwave oven with no ferrite bead is operated under the maximum rated power and under the minimum rated power with a slight load, respectively. As apparent from the curve, a large current flows through the cathode filament of the magnetron just after the magnetron starts the operation. The current in the cathode filament is reduced until the oscillation becomes stable, i.e., during the temperature increase of the magnetron from the room temperature up to the working equilibrium temperature, at which the cathode filament current becomes stable. It is obvious, from FIG. 7, that the cathode filament current I.sub.0 f exceeds the upper limit UL during the initial stage of operation under the maximum rated power in the case where the cathode filament driving coil is directly connected to the magnetron through a connecting wire.
When the cathode filament current I.sub.0 f exceeds the upper limit UL, a lattice defect of the metallic material will be observed in the surface of the cathode filament which causes an abnormal oscillation or a so-called "moding" of the magnetron, resulting in a very short useful life of the magnetron.
On the other hands, the cathode filament current I.sub.1 l falls below the lower limit LL during the operation under the minimum power with a slight load after reaching the working equilibrium temperature, in the case where the ferrite bead is mounted on the connecting wire to provide a predetermined inductance.
When the cathode filament current I.sub.1 l falls below the lower limit LL, there will be a self heat generation by the microwave reflection due to a slight cooling load and insufficient thermal electrons which cause the moding of the magnetron, resulting also in a very short useful life of the magnetron.
Thus, the microwave oven of the art has a problem such that the life time of magnetron is shortened due to the excessive current just after the operation start or to the insufficient current after reaching the working equilibrium temperatures.